1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an apparatus and method for driving a liquid crystal display device, which can selectively provide a wide viewing angle and a narrow viewing angle and improve narrow viewing angle characteristics.
2. Discussion of the Related Art
In general, a liquid crystal display device displays an image by injecting a liquid crystal between two substrates and applying an electric field to the liquid crystal through electrodes facing each other with the liquid crystal interposed therebetween to adjust light transmittance of the liquid crystal.
Such liquid crystal display devices may be classified into a liquid crystal display device of a vertical electric field application type and a liquid crystal display device of a horizontal electric field application type depending on the direction of an electric field applied to drive the liquid crystal.
The liquid crystal mode of the vertical electric field application type is a twisted nematic (TN) mode where the liquid crystal is driven by a vertical electric field between a pixel electrode and a common electrode disposed on a lower and an upper substrate respectively to face each other. In the TN mode, a large aperture ratio can be provided because both the common electrode on the upper substrate and the pixel electrode on the lower substrate forming the vertical electric field are transparent electrodes. However, because the liquid crystal is vertically driven by the vertical electric field, the motion of the liquid crystal has an effect on light traveling laterally. As a result, the viewing angle of the liquid crystal display device is narrowed to about 90°.
The liquid crystal mode of the horizontal electric field application type is an in-plane switching (IPS) mode where the liquid crystal is driven by a horizontal electric field between a pixel electrode and a common electrode arranged in parallel on a lower substrate. In the IPS mode, because the liquid crystal is horizontally driven by the horizontal electric field, there is little vertical motion of the liquid crystal. As a result, the motion of the liquid crystal has little effect on light traveling laterally, so that the viewing angle of the liquid crystal display device is widened to about 160°.
Conventionally, liquid crystal cells formed in a liquid crystal panel of a liquid crystal display device are arranged in a stripe type. However, a liquid crystal display device has been developed which comprises a liquid crystal panel having a quad type cell structure including one electrically controlled birefringence (ECB) sub-pixel and three red, green and blue (RGB) sub-pixels to enable selective switching between a wide viewing angle mode and a narrow viewing angle mode.
As shown in FIG. 1, a liquid crystal cell of a quad type includes a red (R) sub-pixel, a green (G) sub-pixel, a blue (B) sub-pixel, and an ECB sub-pixel, in which the R and G sub-pixels are arranged horizontally and the ECB and B sub-pixels are arranged in parallel with the R and G sub-pixels.
The R and ECB sub-pixels, positioned vertically with respect to each other, are connected in common to a first data line DL1, and the G and B sub-pixels, positioned vertically with respect to each other, are connected in common to a second data line DL2. Also, the R and G sub-pixels, positioned horizontally with respect to each other, are connected in common to a first gate line GL1, and the ECB sub-pixel and B sub-pixel, positioned horizontally with respect to each other, are connected in common to a second gate line GL2.
Here, the ECB sub-pixel is used to coordinate the wide viewing angle mode and the narrow viewing angle mode. In other words, the respective RGB sub-pixels are used to display an original image, and the ECB sub-pixel is used to display an interference image such that the original image is not accurately viewed in a lateral direction of the liquid crystal panel (for example, a direction of about 45° from the front of the liquid crystal panel).
The interference image is also displayed by the ECB sub-pixel while the original image is displayed by the RGB sub-pixels. As a result, the original image and the interference image are simultaneously displayed in the lateral direction of the liquid crystal panel. That is, in the front of the liquid crystal panel, only the original image is viewed and the interference image is not viewed, but in the lateral direction of the liquid crystal panel, an overlap of the original image and the interference image is viewed, thus providing a narrow viewing angle.
However, the conventional liquid crystal panel with the quad type cell structure is disadvantageous in that it has to output a black original image onto a white background image such that only the original image is viewed in the front of the liquid crystal panel. In other words, in order to allow the interference image not to be viewed in the front of the liquid crystal panel and only the original image to be viewed in the front of the liquid crystal panel, there is a monotony of having to display an image of low brightness on a background image of high brightness. Provided that a white image is outputted onto a black background image, an overlap of the original image and the interference image will be viewed even in the front of the liquid crystal panel.
On the other hand, in the case where image boundaries are clearly distinguished by the original image, for example, a black letter or a corresponding image is displayed on a white background, a phenomenon that the brightnesses of the image boundaries are prominently seen may occur even though the interference image is displayed by the ECB sub-pixel. As a result, the original image may be identified at the viewing angle, resulting in difficulty in providing a narrow viewing angle.